[Doc][namespaces][C++ worker]add document for c++ worker namespace and specifying namespace while creating/getting named actors (#26498)
[Doc][namespaces][C++ worker]add document for c++ worker namespace and specifying namespace while creating/getting named actors (#26498)

We've supported namespace in c++ worker in https://github.com/ray-project/ray/pull/26327. Here we add doc for usage and also reinforce the documents of Java and Python, like adding explanation of specifying namespace while creating named actors.

- [x] add doc for basic c++ worker namespace usage
- [x] add explanation for specifying namespace while creating named actors, in Python, Java and C++
ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray